To borrow an exclamation often used by Zippy, "Yikes!"
I just looked on the Remington County Store and they want $84.95 for a "Remington Model 870 18" Magazine Extension Kit (+2Rd, Parkerized) (19420)" http://www.shopremingtoncountry.com/...Rank=salesrank
People say that the Express is not Parkerized but a kind of blued finish, but that is what it looks like imo. I just took a close look at the 870 I bought that came with the 2 round extension and it is hard to say whether the extension is or is not the same finish as the rest of the gun. (I do know that the model I purchased is the same one a guy with a police armory business likes to buy for the one's he builds for police agencies. He takes the new weapon and then parkerizes it to get a better finish that even Remington's parkerized guns.
Too answer your question, if you get the extension from Remington, of the two get the model listed above.
Save yourself $45 and get this on ebay. http://www.ebay.com/itm/Remington-87...item2a1e34c860